1. Field of the Invention
The present invention relates to a process of demodulating received data in a communication technique such as UHF RFID (Radio Frequency Identification) carried out using protocols defined by EPC global, and particularly to a mirror sub-carrier demodulation circuit capable of receiving and demodulating data of a mirror sub-carrier transmitted from a tag and a receiving device including the mirror sub-carrier demodulation circuit.
2. Description of the Related Art
In the past, a method described below (Patent Document 1) was is disclosed as a method of demodulating an RFID. FIG. 16 is a diagram illustrating a hardware configuration of a receiving unit in RFID communication. In FIG. 16, Reference Numeral 1601 denotes an antenna receiving an UHF RFID wireless signal. Reference Numeral 1602 denotes an RF receiving unit which converts a received RF signal into an analog electric signal and includes a band pass filter, an LNA, and the like. Reference Numeral 1603 denotes a down-converter that performs a down-convert from the analog RF signal to an in-phase signal (I signal) and a quadrature signal (Q Signal) of a base-band subjected to the down-convert. The base-band signal subjected to the down-convert has a waveform illustrated in FIG. 17(a). Depending on the configuration of the RF receiving unit 1602, the waveform becomes a sine-like waveform illustrated in FIG. 17(b). Reference Numeral 1604 denotes an AD converter that converts the analog I and Q signals of the base-band signal into a digital signal. Reference Numeral 1605 denotes a demodulation circuit that demodulates data from the digitized reception signal. According to Patent Document 1, the RF receiving unit 1602 receives the wireless signal, the down-converter 1603 converts the analog I and Q signals of the base-band, the AD converter 1604 converts and samples the waveform of the I and Q signals into digital data. The modulation circuit 1605 determines and demodulates data of 0 or 1 in the I and Q signals by the number (length of positive data) of continuous positive data and the number (length of negative data) of continuous negative data in the data sampled as in FIG. 18. When the demodulation result of the I signal is different from the demodulation result of the Q signal, there is disclosed the fact that the result obtained by demodulating a side having a larger signal intensity is used.    Patent Document 1: U.S. Pat. No. 6,501,807
However, in the known modulation method, when a large pulse noise occurs in a much noise environment, there occurs a problem in that the sign of sampling data is changed and there is a high possibility of erroneously determining 1-bit data.
Moreover, since it is necessary to severally demodulate the I and Q signals, there occurs a problem in that the size of a circuit increases.
In an RFID demodulation circuit, a demand for surely demodulating more tag information in any environment increases.